A history of snowfall on Greenland, hidden in ancient leaf waxes

UB researcher Elizabeth Thomas holds half of a sediment core collected from the lake in western Greenland where the study was sited. Such sediment cores contain aquatic leaf waxes that reveal information about the history of precipitation at a site. Credit: Douglas Levere

A surprising trove of data yields indications of increased Arctic snowfall in times of warming

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A new study uses aquatic leaf waxes to study the history of
precipitation at this lake in Western Greenland. Credit: Jason
Briner

BUFFALO, N.Y. — The history of Greenland’s snowfall
is chronicled in an unlikely place: the remains of aquatic plants
that died long ago, collecting at the bottom of lakes in horizontal
layers that document the passing years.

Using this ancient record, scientists are attempting to
reconstruct how Arctic precipitation fluctuated over the past
several millennia, potentially influencing the size of the
Greenland Ice Sheet as the Earth warmed and cooled.

An early study in this field finds that snowfall at one key
location in western Greenland may have intensified from 6,000 to
4,000 years ago, a period when the planet’s Northern
Hemisphere was warmer than it is today.

While more research needs to be done to draw conclusions about
ancient precipitation patterns across Greenland, the new results
are consistent with the hypothesis that global warming could drive
increasing Arctic snowfall — a trend that would slow the
shrinkage of the Greenland Ice Sheet and, ultimately, affect the
pace at which sea levels rise.

“As the Arctic gets warmer, there is a vigorous scientific
debate about how stable the Greenland Ice Sheet will be. How
quickly will it lose mass?” says lead researcher Elizabeth
Thomas, PhD, an assistant professor of geology in the University at
Buffalo College of Arts and Sciences who completed much of the
study as a postdoctoral fellow at the University of Massachusetts
Amherst.

“Climate models and observations suggest that as
temperatures rise, snowfall over Greenland could increase as sea
ice melts and larger areas of the ocean are exposed for
evaporation. This would slow the decline of the ice sheet, because
snow would add to its mass,” Thomas says. “Our findings
are consistent with this hypothesis. We see evidence that the ratio
of snow to rain was unusually high from 6,000 to 4,000 years ago,
which is what you would expect to see if sea ice loss causes
snowfall to increase in the region.”

Aquatic plant leaf waxes as a record of snowfall

The sediment core used in the new study led by UB researcher
Elizabeth Thomas. The column, from a lake bottom in Western
Greenland, contains aquatic leaf waxes that reveal information
about the history of precipitation at the
site. Credit: Jason Briner

Sam Kelley (at the time, a UB geology PhD student, now a
postdoctoral fellow at Waterloo University), left, and
Nicolás Young (at the time, a UB geology PhD student, now an
assistant research professor at Lamont Doherty Earth Observatory),
right, use a saw to slice a sample of lake-bottom mud in half to
prepare the sample for shipment from Western Greenland back to UB.
Credit: Jason Briner

Members of the research team collecting lake sediment cores from
a coring platform in Western Greenland. From left: Stefan
Truex (at the time, a UB undergraduate geology student, now at
Aztech Technologies); Nicolás Young (at the time, a UB
geology PhD student, now an assistant research professor at Lamont
Doherty Earth Observatory); Sam Kelley (at the time, a UB geology
PhD student, now a postdoctoral fellow at Waterloo University).
Credit: Jason Briner

Thomas’ research looks to understand how precipitation
changed in the past, with an eye toward better predicting how
modern warming will affect the Earth.

“We are using the past to see what might happen in the
future,” she says.

Aquatic leaf waxes are a relatively new tool for completing this
work. They reveal information about the seasonality of
precipitation — how amounts of ancient summer rain compared
to amounts of ancient winter snow.

To understand how aquatic leaf waxes function as a historical
record, you need to know a little about aquatic plants. In the
Arctic, these organisms survive on lake water, and use hydrogen
atoms from this water to produce wax coatings on leaves.

These hydrogen atoms are the key to studying precipitation: In
years when the ratio of summer rain to winter snow in a region is
high, lake water and aquatic leaf waxes end up containing high
levels of a rare form of hydrogen called deuterium, which is
heavier than “normal” hydrogen. (This is because summer
rain holds more deuterium than winter snowfall.)

In contrast, in years when snow is relatively abundant, aquatic
plants start producing waxes with less deuterium.

This is what Thomas and her colleagues saw when they extracted a
long, cylindrical sediment sample from a lake bottom in western
Greenland. The mud contains ancient leaf waxes, with the oldest at
the base of the column and the youngest at the top.

By dating and analyzing thin slices of the sample, the team
determined that aquatic leaf waxes had low levels of deuterium from
6,000 to 4,000 years ago.

This is exactly what researchers would expect to see if the warm
temperatures of that time had fueled sea ice loss, leading to
increased Arctic snowfall and a decline in deuterium in lakes,
Thomas said. She acknowledged that it’s possible that a drop
in summer rainfall accounted for the changes in deuterium, but says
a rise in winter snowfall is the more likely explanation, as
scientists have found independent evidence that the region was
wetter 6,000 to 4,000 years ago.

The research team included UB Associate Professor of Geology
Jason Briner; undergraduate student John J. Ryan-Henry from Brown
University, the University of Rhode Island and the Roger Williams
University School of Law; and Professor Yongsong Huang from Brown
University and the Chinese Academy of Sciences. The study was
funded by the National Science Foundation and research grants from
the Geological Society of America.

A new study uses aquatic leaf waxes to study the history of precipitation at this lake in Western Greenland. Credit: Jason Briner